As well known, a glass lens has been used for curing people of various types of abnormal visual acuities like hyperopia, myopia, presbyopia, and so on which are generally caused by the troubles with the lens of an eyeball. In some cases, especially, there is a need for the lens having one or more diopters, and to satisfy the need, is provided multiple focal lens like double focal lens.
For example, as people get older, they suffer from both of the myopia and presbyopia. Therefore, they should have both of the glasses for curing their myopia and the glasses for curing their presbyopia. To eliminate such their inconveniences, there has been developed double focal lens that is made in such a manner as to permit a user to view both of the short and long distances.
The double focal lens is configured in such a manner that two kinds of lenses for uses of long distance and short distance having different refractive indexes are formed of a single lens so as to clearly observe the objects at different distances, so that a user can observe the objects at close distances, without changing his or her glasses, even at the state of clearly viewing the objects at long distances.
The multiple focal lens including the double focal lens first cures a lens's distance portion and then determines reading addition according to the distance lens, thereby inserting the reading lens into the distance lens. In general, the multiple focal lens has the optical central points of the reading and distance portions that are spaced by the left and right length of 2 mm and the height of 10 mm. This is because eyes are converged on a nose and the observing angle from the long distance to the short distance are different when the lens is moved from the distance portion to the reading portion.
On the other hand, a mold for manufacturing the multiple focal lens and the double focal lens is provided with upper and lower support plates 1 and 2 each of which having a single focus and curvature for curing a single distance at the inner peripheral surface thereof, as shown in FIG. 1.
A reference numeral 3, which is not explained in the drawing, denotes a pressure rubber plate.
A conventional double focal lens is manufactured in such a fashion that the distance lens and the reading lens that are separately made by the above-mentioned mold are coupled to provide two optical central points, that is, a distance lens focus and a reading lens focus on the single lens. This causes the manufacturing costs to become undesirably high.
Contrarily, in case where a user reads or works with a computer, he or she reads at a relatively close distance but watches the monitor at a relatively long distance. In this case, the two distances are somewhat different. At this time, if he or she is wearing generally reading glasses having a single diopter, he or she cannot view very well the monitor at a longer distance than the book, such that he or she feels his or her eyes get tired and also his or her posture becomes poor.
Therefore, there is a need for improvement of the single reading lens of clearly viewing all of the near point objects and the far point objects. In case of the conventional multiple focal lens from the viewpoint of applying the need for the improvement, however, it has provided the following problem:
If the optical central points are different, the objects at the respective focuses look positioned differently, and the phases of the objects look scattered by the positions of the optical central points when the observing directions are moved shortly in upward and downward directions while seeing short distances within the same distance.